There is known a projection type image display apparatus such as a liquid crystal projector wherein light emitted from a light source such as an electric lamp is directed to an image display element such as a liquid crystal panel and an optical image formed on the image display element is projected on a larger scale. In this type of a projection type image display apparatus, light emitted from a light source is subjected to light intensity modulation for conversion into light and shade for each pixel in an image display element and an optical image formed is projected to a screen disposed on the front side or from the back side of a screen thereto. Such an image display apparatus is described, for example, in Japanese Patent Laid-open Nos. 10-171045 and 11-281923.
The mainstream of a light source used in the projection type image display apparatus is such that light emitted from the lamp is reflected by a reflector 2 which is disposed so as to cover the lamp from the back side and is then outputted to the image display element side. Such a light source, however, provides a light ray component that is not incident on the reflector but is radiated from an opening side of the reflector. Thus, satisfactory light utilization efficiency is not obtained. Such a loss can be compensated for by making the reflector large in size. In this case, however, an increase in size of the light source, i.e., an increase in size of the projection type image display apparatus, results and it is impossible to meet the market demand for reduction in size.
As means for attaining the reduction of size while improving the light utilization efficiency, a method (hereinafter referred to as the “double reflector method”) is described in, for example, Japanese Patent Laid-open No. 6-289394 wherein a second reflector is disposed in the direction of light radiated from the reflector opening side and in proximity to a lamp.
The light source used in the double reflector method disclosed in Japanese Patent Laid-open No. 6-289394 includes a lamp, a reflector which covers the lamp from the back side and which reflects light incident from the lamp to an image display element side (opening side), and a sub-reflector disposed in proximity to the front side of the lamp. The sub-reflector reflects light incident from the lamp to the reflector side. For example, the sub-reflector may be a reflective film formed on an outer surface of a tubular bulb located on the side opposite to the reflector side of the lamp.
Light rays in the double reflector type are classified broadly into two, which are a light ray L3 incident on the reflector from the lamp and goes out after being reflected by only the reflector and a light ray L6 incident on the sub-reflector from the lamp and goes out after being reflected by the sub-reflector and subsequently reflected by the reflector. The light ray L6 is further classified into two light rays L7 and L8. L7 is a light ray which, in the absence of the sub-reflector, does not enter the reflector, but radiates from the opening portion of the reflector without advancing toward (without becoming incident on) integrator means, the integrator means being for uniforming a light quantity distribution, or even after being incident on and reflected by the reflector, doe not advance toward (does not become incident on) the integrator means and thus becomes invalid. L8 is an effective light ray which, even in the absence of the sub-reflector, is reflected by the reflector and advances toward (becomes incident on) the integrator means. Thus, L7 is a light ray which can be utilized effectively by adopting the double reflector method. In the following description it is assumed that the light ray 3 includes a light ray that, in the absence of the sub-reflector, enters the reflector, then is reflected by the reflector and advances toward the integrator means.